Method and system to improve the transport of compressed video data

ABSTRACT

A method comprises determining a plurality of time intervals Tp and Tn within a variable bit rate (VBR) representation of an image sequence. The time intervals Tp are those in which a number of blocks of information per unit time is greater than a baseline value. The time intervals Tn are those in which a number of blocks of information per unit time is less than the baseline value. A second representation of the image sequence is created in which some blocks of information Bp are removed from the time intervals Tp and interlaced with blocks of information Bn in the time intervals Tn to reduce a variation in a number of blocks of information per unit time between the time intervals Tp and Tn.

RELATED APPLICATIONS

The present patent document is a continuation of U.S. patent applicationSer. No. 10/893,195, filed Jul. 16, 2004, now U.S. Pat. No. 7,054,368which is a divisional of U.S. patent application Ser. No. 09/942,260,filed Aug. 28, 2001, now U.S. Pat. No. 6,970,512 the entirety of whichare both hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to methods and systems for improving thetransport of variable bit rate data signals over a bandwidth limitedcommunication network.

BACKGROUND

Numerous compression schemes address the transport and reconstruction ofmotion images (e.g. video) for pseudo-real-time and non-real-timeapplications. Many of these schemes make use of buffers, especially at areceiving end of a communication network, for storing partial blocks ofinformation which are pre-transmitted to the receiver. Forpseudo-real-time applications, the buffer has a buffer length which is afunction of a total amount of bits of information to be sent and abandwidth available in the communication network. For non-real-timeapplications, part of the information, such as Discrete Cosine Transform(DCT) coefficients, is sent ahead of time, while the rest of theinformation is sent later and reconstructed in real time.

The Motion Pictures Experts Group 2 (MPEG2) compression standard makesuse of motion compensation to reduce the data rate. Although the contentis compressed at a certain bit rate, such as 1.5 Megabits per second(Mbps), the actual bandwidth used temporally varies. The temporalvariation creates peaks and troughs in the bandwidth. For purposes ofillustration and example, consider a hypothetical real-time transmissionof compressed motion images which produces a bit rate versus time graph10 shown in FIG. 1. The bit rate has an upper bound of 1.5 Mbps and isvariable over time. In a DVD movie, for example, the bit rate may varyfrom 2.5 Mbps to 8 Mbps.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims.However, other features of the invention will become more apparent andthe invention will be best understood by referring to the followingdetailed description in conjunction with the accompanying drawings inwhich:

FIG. 1 is a graph of bit rate versus time for a hypothetical real-timetransmission of compressed motion images;

FIG. 2 is a flow chart of an embodiment of a method of improving thetransport of compressed video data;

FIG. 3 illustrates the transmission curve of FIG. 1 in terms of blocksof information that are sent per unit time;

FIG. 4 is a flow chart of an embodiment of a method performed at areceiver; and

FIG. 5 is a block diagram of an embodiment of a system to perform theherein-disclosed methods.

DETAILED DESCRIPTION OF THE DRAWINGS

Disclosed herein are methods and systems that can improve, andoptionally optimize, the video quality of bandwidth-limited transmissionlinks. By analyzing content in advance of final coding, a constant bitrate (CBR) or a near-CBR type data stream can be constructed thatencodes video at a higher bit rate than existing coding schemes. Theresult is a higher quality video delivery on the same band-limited link.

FIG. 2 is a flow chart of an embodiment of a method of improving thetransport of compressed video data. As indicated by block 20, the methodcomprises encoding an image sequence to provide a variable bit rate(VBR) representation thereof. The encoding may be based upon apre-selected peak bit rate which the VBR representation is not toexceed. The image sequence may be encoded in accordance with an MPEGcompression standard such as MPEG2, for example. The resulting VBRrepresentation comprises a plurality of blocks of information.

For purposes of illustration and example, consider that the resultingVBR representation has the transmission curve given in FIG. 1. FIG. 3illustrates the transmission curve of FIG. 1 in terms of blocks ofinformation that are sent per unit time. FIG. 3 considers thetransmission curve of FIG. 1 from an energy perspective. The power overa time segment is based on an integral of the transmission curve overthe time segment. Further, the instantaneous value varies based on theamplitude of the curve at a point in time. During complex scenes withsignificant motion, the number of blocks of information is relativelyhigh. In contrast, during periods of little or no motion, the number ofblocks of information is relatively low.

Referring back to FIG. 2, an analysis of block coding statistics isperformed as indicated by blocks 22, 24 and 26. In particular, block 22indicates an act of determining a plurality of time intervals Tp withinthe VBR representation in which a number of blocks of information perunit time is greater than a baseline value. Block 24 indicates an act ofdetermining a plurality of time intervals Tn within the VBRrepresentation in which a number of blocks of information per unit timeis less than the baseline value. Referring back to FIG. 3, the baselinevalue is indicated by reference numeral 28, the plurality of timeintervals Tp are indicated by reference numeral 30, and the plurality oftime intervals Tn are indicated by reference numeral 32. The baselinevalue 28 may be based on an average value for the curve. The baselinevalue 28 represents the bit rate desired when the transmission rate hasbeen chosen.

In the context of this application, the variable Bp represents theequivalent block data that resides above the baseline value in a Tp timeinterval. The variable Bn represents the equivalent block data thatresides below the baseline value in a Tn time interval. Block 26 in FIG.2 indicates an act of calculating a sum of Bp and Bn information toensure that ΣBn≧ΣBp. Optionally, this act may include increasing thebaseline value 28 from the average value to ensure that ΣBn≧ΣBp. Asanother option, the baseline value 28 may be determined such thatΣBn=ΣBp, which provides an optimal condition for the present invention.

As indicated by block 34, an act of determining if a desired maximuminformation content is attained. This act may comprise determining ifthe baseline value is less than or equal to a threshold value, such asthe bandwidth limit of a communication network.

If the desired information content is not attained, flow of the methodis directed back to block 20 wherein the image sequence is re-encodedfor a higher peak bit rate to form another VBR representation. The actsindicated by blocks 22, 24 and 26 are repeated to analyze the blockcoding statistics for the new VBR representation. The acts indicated byblocks 20, 22, 24 and 26 are repeated until a desired maximuminformation content is attained or exceeded.

Once the desired information content is attained, an act of creating asecond representation of the image sequence is performed as indicated byblock 36. In the second representation, some blocks of information Bpare removed from the time intervals Tp, and time-advanced to beinterlaced with blocks of information Bn in the time intervals Tn toreduce a variation in a number of blocks of information per unit timebetween the time intervals Tp and Tn. To create the secondrepresentation, the Tp and Tn time intervals are tagged. The timeintervals may be tagged based on a frame number. It is then determinedwhere time-advanced Bp blocks can be inserted into Tn time intervals.Preferably, the time-advanced Bp blocks are distributed into Tn timeintervals so that the number of blocks of information per unit time inthe second representation is about equal to the baseline value in all ofthe time intervals Tp and Tn. In an exemplary case, the secondrepresentation is a CBR representation in which the number of blocks ofinformation per unit time in the second representation is equal to thebaseline value in each of the time intervals Tp and Tn. A file iscreated which comprises sequential real-time Bp and Bn interlaced blockdata.

As indicated by block 40, an act of determining buffer requirements at atransmitter is performed. As indicated by block 42, an act of populatinga header in the second representation with data indicating the timeintervals Tn and the buffer requirements. Preferably, the header ispopulated with the length and number of Tn time intervals.

As indicated by block 44, an act of streaming the second representationof the image sequence via a communication network is performed. Thesecond representation comprises the header and the file. Referring backto FIG. 3, the bit rate versus time of the resulting data stream isillustrated by the curve indicated by reference numeral 50. Note that inthe Tn time intervals, real-time Bn block information along withtime-advanced Bp block information is transmitted. The resulting streamin this example is a CBR stream which conforms to the link rate of 1.5Mbps, but in essence contains coded video at a higher rate, such as 2.0Mbps for example.

Beneficially, the acts indicated by blocks 20, 22, 24, 26 and 34 may beused to determine a bit rate for encoding the image sequence to the VBRrepresentation which produces a desired information content of thesecond representation and constrains a maximum bit rate of the secondrepresentation to be less than or equal to a predetermined value.Optionally, the aforementioned acts may be used to determine a bit ratefor encoding the image sequence to the VBR representation whichsubstantially maximizes a desired information content of the secondrepresentation and constrains a maximum bit rate of the secondrepresentation to be less than or equal to a predetermined value.

FIG. 4 is a flow chart of an embodiment of a method performed at areceiver. As indicated by block 52, the method comprises receiving thesecond representation of the image sequence via the communicationnetwork. As indicated by block 54, the buffer requirement data and theTn parameters are extracted from the header. Based on the bufferrequirement data, a buffer is provided for storing Bp block information(block 56). Preferably, the buffer comprises a content addressablememory (CAM) type buffer.

Frames of the image sequence are reconstructed concurrently with thesecond representation being received. During the time intervals Tn,frames of the image sequence are reconstructed based on blocks ofinformation Bn received about in real time (block 57). Further duringthe time intervals Tn, the blocks of information Bp which are receivedare stored in the buffer (block 58). During the time intervals Tp,frames of the image sequence are reconstructed based on the blocks ofinformation Bp stored in the buffer and blocks of information receivedabout in real time (block 59).

As used herein, the phrase “about in real time” contemplates anyprocessing and/or storage delays which may result in a non-strict realtime reconstruction of the frames. Thus, the frames of the imagesequence are reconstructed concurrently with the reception of the secondrepresentation either strictly in real time or non-strictly in realtime.

FIG. 5 is a block diagram of an embodiment of a system to perform theherein-disclosed methods. An encoder 60 encodes an image sequence 62 toprovide a VBR representation 64. A processor 66 performs the blockcoding statistics analysis of the VBR representation 64. The processor66 may direct the encoder 60 to re-encode the image sequence 62 based onthe aforementioned analysis until a desired information contentcondition is attained.

The processor 66 creates a file 70 that contains a representation of theimage sequence 62 in which some blocks of information Bp are removedfrom the time intervals Tp and interlaced with blocks of information Bnin the time intervals Tn to reduce a variation in a number of blocks ofinformation per unit time between the time intervals Tp and Tn. Theprocessor 66 populates a header 72 with data indicating the timeintervals Tn. The file 70 and the header 72 are stored by acomputer-readable storage medium. A transmitter 74 streams the header 72and the file 70 via a communication network 76.

The system comprises a receiver 80 to receive the header 72 and the file70 via the communication network 76. A processor 82 is responsive to thereceiver 80 to extract data indicating the time intervals Tn from theheader 72. The processor 82 reconstructs frames of the image sequenceconcurrently with the reception of the file 70. During the timeintervals Tn, the processor 82 reconstructs frames of the image sequencebased on blocks of information Bn received about in real time. Furtherduring the time intervals Tn, the processor 82 stores the blocks ofinformation Bp in a buffer 84. During the time intervals Tp, theprocessor 82 reconstructs frames of the image sequence based on theblocks of information Bp stored in the buffer 84 and blocks ofinformation received about in real time. Reconstructed frames of theimage sequence are indicated by reference numeral 86.

The acts performed by the processor 66 may be directed bycomputer-readable program code stored by a computer-readable medium.Similarly, the acts performed by the processor 82 may be directed bycomputer-readable program code stored by a computer-readable medium.

Preferred embodiments of a method and system to improve the transport ofcompressed video data have been described herein. The embodimentsdisclosed herein facilitate higher bit rate content to be transmittedover the same band-limited transmission link. Making use of MPEG2 blockstructure and block sequence reduces the computational complexity of thescheme and is well suited to CAM-oriented silicon solutions.

It will be apparent to those skilled in the art that the disclosedinvention may be modified in numerous ways and may assume manyembodiments other than the preferred form specifically set out anddescribed above.

Accordingly, it is intended by the appended claims to cover allmodifications of the invention which fall within the true spirit andscope of the invention.

1. A method comprising: providing a variable bit rate (VBR)representation of an image sequence, the VBR representation comprising aplurality of blocks of information; determining a first plurality ofblocks of information per unit time that is greater than a secondplurality of blocks of information per unit time; and creating a secondrepresentation of the image sequence in which some of the firstplurality of blocks of information are interlaced with blocks ofinformation of the second plurality of blocks of information to reduce avariation in a number of blocks of information.
 2. The method of claim 1wherein the number of blocks of information per unit time in the secondrepresentation is about equal to a baseline value.
 3. The method ofclaim 1 further comprising: determining a bit rate for encoding theimage sequence to the VBR representation which produces a desiredinformation content of the second representation and constrains amaximum bit rate of the second representation to be less than or equalto a predetermined value.
 4. The method of claim 1 further comprising:determining a bit rate for encoding the image sequence to the VBRrepresentation which substantially maximizes a desired informationcontent of the second representation and constrains a maximum bit rateof the second representation to be less than or equal to a predeterminedvalue.
 5. The method of claim 1 further comprising: populating a headerin the second representation with data indicating which blocks ofinformation of the first plurality of blocks of information areinterlaced with blocks of information of the second plurality of blocksof information.
 6. The method of claim 1 further comprising: streamingthe second representation of the image sequence via a communicationnetwork; receiving the second representation of the image sequence viathe communication network; and reconstructing frames of the imagesequence concurrently with said receiving, said reconstructingcomprising: during a period of time when the received stream comprisesblocks of information of the first plurality of blocks of informationinterlaced with blocks of information of the second plurality of blocksof information, reconstructing frames of the image sequence based onblocks of information of the second plurality of information receivedabout in real time, and storing blocks of information of the firstplurality of blocks of information in a buffer; and during a period oftime when the received stream does comprises blocks of information ofthe first plurality of blocks of information only, reconstructing framesof the image sequence based on blocks of information of the firstplurality of blocks of information stored in the buffer and blocks ofinformation received about in real time.
 7. A computer-readable storagemedium encoded with computer executable instructions to direct acomputer system to perform acts of: providing a variable bit rate (VBR)representation of an image sequence, the VBR representation comprising aplurality of blocks of information; determining a first plurality ofblocks of information per unit time that is greater than a secondplurality of blocks of information per unit time; and creating a secondrepresentation of the image sequence in which some of the firstplurality of blocks of information are interlaced with blocks ofinformation of the second plurality of blocks of information to reduce avariation in a number of blocks of information.
 8. The computer-readablestorage medium of claim 7, wherein the number of blocks of informationper unit time in the second representation is about equal to a baselinevalue.
 9. The computer-readable storage medium of claim 7 furthercomprising a set of instructions to direct the computer system toperform acts of: determining a bit rate for encoding the image sequenceto the VBR representation which produces a desired information contentof the second representation and constrains a maximum bit rate of thesecond representation to be less than or equal to a predetermined value.10. The computer-readable storage medium of claim 7 further comprising aset of instructions to direct the computer system to perform acts of:determining a bit rate for encoding the image sequence to the VBRrepresentation which substantially maximizes a desired informationcontent of the second representation and constrains a maximum bit rateof the second representation to be less than or equal to a predeterminedvalue.
 11. The computer-readable storage medium of claim 7 furthercomprising a set of instructions to direct the computer system toperform acts of: streaming the second representation of the imagesequence via a communication network; receiving the secondrepresentation of the image sequence via the communication network; andreconstructing frames of the image sequence concurrently with saidreceiving, said reconstructing comprising: during a period of time whenthe received stream comprises blocks of information of the firstplurality of blocks of information interlaced with blocks of informationof the second plurality of blocks of information, reconstructing framesof the image sequence based on blocks of information of the secondplurality of information received about in real time, and storing blocksof information of the first plurality of blocks of information in abuffer; and during a period of time when the received stream doescomprises blocks of information of the first plurality of blocks ofinformation only, reconstructing frames of the image sequence based onblocks of information of the first plurality of blocks of informationstored in the buffer and blocks of information received about in realtime.